To study the modulating effect of polysaccharide and protein secreted by Shaerotilus upon the mineralization behaviors of Fe(OH)3 gel, the authors investigated the nucleation and crystallization processes of Fe(OH)3gel inside and around the Sphaerotilus cells which were growing in the mineralization systems with urea and Stoke substrate. The minerals were characterized by TEM, UV and FTIR spectrometry. A comparison of the mineralization processes of Fe(OH)3 gel functioned by pure dextran shows that both microbial polysaccharide and dextran could control the nucleation mechanisms, nuclei phases and the growth rate of ferric ions and Fe(OH)3gel in the solution. The major mineral seems to be nanometric size akaganeite (β-FeOOH).